The present invention relates generally to electric motor systems. More specifically, various embodiments of the present invention are directed to monitoring, determination, and control of improper rotor operation, which include, but not limited to, oscillatory state and/or locked state of rotors. Among others, embodiments of the present invention reduce and/or eliminate undesirable effects of motor oscillation states.
Electric motor systems have been widely used since Michael Faraday demonstrated the principle of conversion of electrical energy into mechanical energy in 1821. Today, from car engines to computer components, electric motor systems have a wide range of applications.
Many electronic devices that require efficiency, performance, or control use brushless DC motors. These electronic devices include computer fans, optical drives, etc.
Brushless DC motors generally comprise three major stages: a control stage, a pre-driver stage, an actual driver stage. A typical actual driver stage can be bipolar or unipolar. In some applications, there are multiple-pole brushless motors that use a polyphase driver stage. A bipolar driver stage comprises four switching devices, e.g., field effect transistors (FETs) or bipolar junction transistors (BJTs), arranged in a full-bridge configuration. The switching devices are driven by complementary pulses generated by the pre-driver stage such that the switching devices that are located diagonally with respect to one another are turned on at the same time. A unipolar driver stage consists of two switching devices arranged in a half-bridge configuration, only one of which is turned on at one time.
The pre-driver stage consists of a discrete integrated circuit (IC) that generates the complementary pulses for the driver stage in response to the output from a Hall sensor. For example, in a fan, the Hall sensor is switched by the permanent magnet component of a rotor of a motor. When one magnetic pole of the permanent magnet comes near the Hall device as the rotor rotates, the output of the Hall sensor switches from one logic state (e.g., a logic low or a logic high) to the complementary logic state. The output of the hall sensor switches back when the opposite polarity is sensed by the Hall sensor. The switching of the Hall sensor provides angular position information of the rotor. Usually, the angular position from the Hall sensor is sent to and used by the control IC.
FIGS. 1 and 2 show a simplified block diagram and a schematic diagram respectively of a brushless DC motor system. Detailed descriptions on the operation of microcontroller-operated DC motor are provided U.S. Pat. No. 6,611,117, titled “DC Circuit for a Brushless DC Motor”, commonly owned by the assignee the present invention and incorporated herein in its entirety for all purposes. The motor 100 includes a Hall sensor 10 having an output 12; a microcontroller 20 having complementary outputs 30 and 40; stator coil 50; and switches SW1 and SW2. In the block diagram shown in FIG. 1, the switches SW1 and SW2 comprise the two switches that are on at the same time in a full-bridge driver stage. In the schematic diagram shown in FIG. 2, the switches SW1 and SW2 of FIG. 1 are represented by switches 60 and 70 or switches 80 and 90. An example of Hall sensor 10 is a part commonly known in the industry by part number UA3175 and likewise an example of the microcontroller 20 is a part known in the industry by part number PIC12C671.
One application for the brushless DC motor shown in FIGS. 1 and 2 is in a fan of the type used for cooling electronic circuits. Such a brushless DC fan, which is to say a fan driven by a brushless DC motor, further includes an impeller coupled to the motor and mounted in an impeller housing (not shown). The impeller of the fan is caused to rotate when current flows through the switch SW1, the stator coil 50, and the switch SW2. The direction of impeller rotation, i.e., clockwise or counter-clockwise, is determined by the direction of current flow through the switch SW1, the stator coil 50, and the switch SW2.
In an exemplary implementation of Hall sensor, the impeller housing contains a permanent magnet which produces a magnetic field for the brushless DC fan. The Hall sensor 10 detects a change in the state of the magnetic field that is produced as the impeller of the brushless DC fan rotates in relation to the permanent magnet. As the impeller reaches a rotational extreme in either the clockwise or the counter-clockwise direction, the Hall sensor 10 detects the change in the state of the magnetic field of the brushless DC fan, and the output 12 of the Hall sensor changes its logic state.
The output 12 of the Hall sensor 10 is provided to the microcontroller 20, and the state of the outputs 30 and 40 of the microcontroller 20 is a function of the output 12 of the Hall sensor 10. Thus, according to an embodiment of the present invention, whenever the microcontroller 20 senses a change in the output 12 of the Hall sensor 10, the microcontroller 20 changes its outputs 30 and 40 in a complementary manner. For example, if the output 12 of the Hall sensor 10 is a logic high, the microcontroller 20 causes the output 30 to transition from a logic low to a logic high and simultaneously causes the output 40 to transition from a logic high to a logic low. It will be appreciated by those having skill in the art that the particular relationship between the state of the outputs 30 and 40 of the microcontroller 20 and the output 12 of the Hall sensor 10 can be varied to conform to the requirements of a particular brushless DC motor or fan.
The brushless DC motor described in FIGS. 1 and 2 is more reliable and efficient than older motors due to the presence of a microcontroller. Among other features, the DC motor in FIGS. 1 and 2 provides mechanisms through the microcontroller (and the advanced control algorithm therein) and the Hall effect sensor to prevent the motor from damaging itself from a condition known as the “locked rotor” condition.
A “locked rotor” condition can arise when a motor is “locked” for various reasons, such as undesirable physical interference, unbalanced driving energy, etc. For example, a DC motor that is used for rotating a fan may be locked into a position when the fan is blocked. The closed loop from the output of the driver stage to the pre-driver stage enables the fan to run essentially self-sufficiently. However, there are some conditions where the fan requires assistance to operate correctly and, very importantly, safely. For example, in a “locked rotor” condition, where the fan impeller is stopped for any reason, the fan has to turn itself off in order not to burn out the switching devices in the driver stage. After a predetermined time period of t seconds the fan must determine whether the fan impeller is free to resume rotating. The fan does this by turning on one output of the pre-driver stage and waiting for the impeller to turn. If the impeller does not begin to turn within a predetermined time period, the output of the pre-driver is turned off again. The fan repeats this cycle every t seconds. The timing for the restart cycles is provided by a resistor-capacitor network that is external of the pre-driver IC.
The “locked rotor” situation is not the only pitfall that can potentially cause motors to malfunction. Another problem that can potentially lead to motor malfunction is the oscillatory state problem. In an oscillator state, which is sometimes referred to as oscillatory or “rocking” state, an electric motor lock itself in an angular position in which positive and negative torque is produced, provoking an oscillatory mode that can be described as “rocking”. Typically, a motor goes into an oscillatory state when the rotor is not rigidly held. For example, the motor may be loosely stuck due to a foreign object that is impeding the free wheeling of the motor. If the motor is in this oscillatory mode, it may never get out of it. If the driving module tries to overcome by increasing the driving energy, the increasing current going through the coil and switching devices of the motor can cause the switching devices to burn out.
In the past, various types of conventional techniques have been proposed to address this problem. In some conventional systems, a sensor is provided to monitor movement of the rotor, which may be angular frequency and/or edges from a Hall sensor, when a motor is powered. The conventional system determines whether the rotor is in an oscillatory state based on its movements. Unfortunately, conventional techniques such as the one described above are often inadequate.